Table of contents

Two-dimensional measurements were performed of the relation between the charge transported and number of photons emitted, accompanying discharges in air at the surface of a dielectric. The creep discharges were analysed for opaque dielectrics placed in an AC field of v=50 Hz and intensity of 10³ V mm^-1. A linear relation was found to exist between the number of photons and the charge, N_ph= kappa Q, the coefficient kappa varying with the polarity of the electrode at which the discharges occurred, kappa ⁺/ kappa ^-=0.5-0.8. It is postulated that the difference in kappa ⁺ and kappa ^- is due to the different average energy of electrons giving the discharge channel.

H-like and He-like Al lines and their satellites are obtained from Al and polyethylene layered targets irradiated by a Nd:glass laser at a power flux of 2*10¹³ W cm^-2 on target. Two-dimensional spatial resolution is achieved using a flat crystal spectrograph incorporating a wire shadow technique. Expansion profiles in the lateral direction as a function of the Al layer location are measured and interpreted in terms of the temporal history of the interaction. By varying the location of the Al layer, the penetration depth of the heat wave front is also determined. It is found that in this power flux range, heat conduction can be explained by classical mechanisms.

A novel method for frequency controlling two or more lasers to a high degree of stability, suitable for a low-frequency heterodyning experiment in normal laboratory conditions is described. The method made use of a principal 'local oscillator laser', frequency controlled to 'line-centre', to which two further lasers were controlled without modulation. Electron densities in the range 10¹²-10¹⁵ cm^-3 have been measured in pulsed hydrogen arcs in diode and triode devices at pressures circa 500 m Torr, with a small experimental uncertainty and a temporal resolution of the order of 1 mu s.

Electron energy distributions have been measured in front of and behind a stationary plasma sheath in a low-pressure mercury discharge. The sheath appears at a constriction of the discharge tube. The measurements have been done with a small spherical probe, using the second-derivative method. The results show that the energy distribution of the anode side of the sheath is a sum of a maxwellian and an accelerated distribution. Near the sheath the accelerated electrons suffice to carry the discharge current, but far from it the current requires anisotropy in the thermal part of the distribution function.

X-ray radiation from the plasma generated on a solid target by a subnanosecond laser pulse is investigated. The analysis is a semiquantitative one, based upon the energy equation with the relevant microscopic processes taken into account. For the simple case of a rectangular pulse, there are three different phases: a heating transient stage, a stationary regime in which all absorbed radiation is reradiated as soft X-rays and a cooling phase. The conditions to be fulfilled in order that the expansion be neglected are investigated in detail. These analytical estimates are compared with the results of a computer code. In a model where energy propagates from the laser absorption layer into the target by heat conduction, the conversion efficiency at moderate fluxes is drastically reduced by the expansion which sets on before the regime is reached.

The initial stages of a 40 kJ theta pinch in hydrogen at 13 mTorr have been investigated using a 10.6 mu m TEA CO₂ laser. Electron density profiles measured across the tube using single-fringe interferometry show two imploding density fronts with a region of increased ionization at the tube axis when the first front approaches the axis. Electrons are swept up mainly by the outer front. 10.6 mu m Faraday rotation measurements of the magnetic field on the axis during the implosion stages are described. During the initial stages, the magnetic field is reversed with magnitude increasing to 0.4+or-0.01 Wb m^-2 but changes direction at t=410 ns when the first density front approaches the axis.

The mechanisms of discharges at the surface of a dielectric have been investigated near the inception voltages. The method is based on observations of flash intensity and amplitude as a function of voltage, phase shift and alternating potential frequency. Three ranges of voltage applied were distinguished for three different discharge situations: the relevant discharge mechanisms are discussed.

Framing shadowgraphs have been used to investigate the interaction of a 1.5 GW CO₂ laser with polythene pellets in vacuo. A Spectra-Physics argon-ion laser was used as source and a Hadland Imacon camera for recording, at framing rates of 10⁵-10⁷ Hz. Part of the pellet ionized, and the remainder fragmented. Theoretical analysis suggests that fragmentation can be avoided if the laser pulse and pellet geometry are mutually optimized; optimal parameters are estimated.

In high-current arc discharges the plasma flow velocity distribution is believed to have a very strong spatial dependence (Barrault et al., 1974) and the well established technique of laser velocimetry is apt to behave incorrectly. The extension of the laser scattering technique, which has been mainly used in controlled thermonuclear fusion research is described. The method offers a new velocimetry technique which can offer pinpoint (1 mm³ order) well time-resolved (10¹ ns) diagnostics of the plasma flow velocity.

The decay of the electron density is measured during extinction of an arc discharge at atmospheric pressure. Measurements are made by spectroscopic methods. Complete local thermodynamic equilibrium is not attained for an electron density lower than 3*10¹⁶ cm^-3. The apparent recombination coefficient for electrons and ions is studied as a function of temperature between 10000 and 13000K.

The guiding centre motion of a charged particle in an axisymmetric magnetic field configuration is completely determined by the invariants of motion which represent conservation of energy, canonical angular momentum about the axis of symmetry, and the magnetic moment associated with rotation about magnetic field lines. Using these invariants of motion, it is possible to construct Vlasov equilibrium distributions of ions and electrons and to calculate a self-consistent electrostatic potential in the plasma. Results obtained for the Levitron magnetic containment device at Culham Laboratory are presented.

The expansion of plasmas produced by a mode-locked Nd:glass laser has been studied. With help of the second harmonic radiation of the laser itself the shock waves in the initial stages after breakdown are recorded. With a streak camera the time development of the plasma depending on the gas pressure and the focal length of the focusing lens was studied in He, Ar, Kr, Xe, N₂, CO₂ and SF₆. While the shock waves obey the well known relations for ideal shock waves, the stepwise plasma expansion towards the lens can be described by a simple ionization model.

Measurements are described on a planar sheath established in a moving flame plasma seeded with potassium. By varying the quantity of seed the ionization density of the flame is altered so as to allow a range of sheath current-voltage characteristics which vary from saturation at low densities to a substantial voltage dependence at high densities. When the sheath is suddenly collapsed the ionization within what was the sheath region is observed to recover towards its equilibrium value. Measurements of current density, ionization density, and sheath thickness taken during these experiments show good agreement with a sheath model in which convection and/or ionization regeneration act as the supply of ions to the sheath.

The diffusion coefficient of an ionized gas in the presence of a magnetic field and the dynamical equilibrium of an ion sheath are considered. In the first case diffusion results in enrichment with respect to the heavy element; in the second case separation effects appear on the surface where the voltage is modulated at a frequency larger than the inverse of the relaxation time for equilibrium.

The transport and thermodynamic properties of a copper-nitrogen (Cu-N) mixture are calculated for temperatures between 2000 and 28000K and pressures between 1 and 10 atmospheres. The results show that, compared with the electrical conductivity for pure nitrogen, a small concentration of copper vapour can greatly enhance the electrical conductivity at temperatures below about 8000K. The internal energy and enthalpy per unit mass are reduced by ¹/₂ and ²/₃ when X is increased from 0 to 1, where X is the ratio of the total number density for all copper species to that for all nitrogen species. Changes in the electron thermal conductivity increase the total thermal conductivity by about (1+X) when X<0.1, for temperatures between 9000 and 12000K.

The prebreakdown current growth between gold-plated parallel plane electrodes in hydrogens is measured for 76 Td<E/N<576Td and analysed by fitting the Townsend equation and the Lucas equation. The results suggest that the current growth for E/N<or approximately=310Td, where the photon ( delta _p) and positive ion ( gamma _i) secondary actions are appreciable, is represented better by the Lucas equation. At higher E/N, the growth becomes equally well represented by both equations because the gamma _i action tends to prevail over the delta _p.

Electron and radiation transport of energy is treated by self-similar methods in aluminium targets exposed to extreme radiation intensities. Detailed numerical solution of this phenomenon are given including the effects of incomplete ionization on the conduction processes and the internal energy.

The emission of microparticles in a vacuum gap under the action of an electric field has been studied. Electrodes were prepared by abrasive polishing and an electrostatic detector located behind a grid constituting the cathode was used. The charges and sizes of microparticles depend on the conditioning state of the electrodes. The ratio F of the electrical charge of an actual microparticle to the charge of a hypothetical metal sphere with the same mass lying on a plane, called 'the over-charge factor', ranges from 0.2 to 10. Usually it decreases when the size of the particles increases.

Presents an investigation of the effect of the saturating nonlinearity of the pump wave on the spatial growth of the filamentation instability of a Gaussian plane electromagnetic beam in a collisionless magnetoplasma. The nonlinearity is caused by the redistribution of carriers by the ponderomotive force. The growth rate is not a monotonically increasing function of the beam intensity and the optimum size of perturbation increases with the beam intensity beyond a certain value of the beam intensity which is in contrast to the uniform plane beam case. The growth rate decreases with the distance of propagation due to the self-focusing effects and is enhanced with the increase of static magnetic field for extraordinary waves, but the reverse is true for ordinary waves.